WHO painted The Skating Minister? It seems a simple enough question and for a long time there was a simple enough answer to it. It was Sir Henry Raeburn, the pre-eminent portrait painter of the Scottish Enlightenment, whose name his aficionados confidently couple with Goya's. They say Raeburn painted the Rev Robert Walker skating on Duddingston Loch on the outskirts of Edinburgh in the midst of an era when men of genius, such as David Hume and Adam Smith, roused Voltaire to declare that "today it is from Scotland that we get rules of taste in all the arts, from epic poetry to gardening".

Raeburn, who was born in 1756, painted many of the great and the good of his time, including Sir Walter Scott, who invented the bestseller, and James Hutton, the founder of modern geology. Robert Walker was not in their league, but he was a member of the same select Edinburgh society. We know for certain that Walker and Raeburn were acquaintances. Raeburn, for example, was one of the nine trustees of the minister's will. Nor was Walker's family in any doubt as to who painted their forebear.

Writing in 1926, Miss Beatrix Scott, his great granddaughter, said: "I have always understood that Raeburn considered it his masterpiece, the pose being so good and the lovely frosty atmosphere and the sky and the ice with all the marks of the skates.

Dr Walker was a great skater. On his death, Sir Henry Raeburn gave the picture to his widow, my great grandmother. After her death it came to my mother."

That would appear to be that. But nothing could be further from the truth. Over the past year, a debate which had been previously confined to art historians and conducted in whispers suddenly erupted into the public domain and made headline news. It was detonated by Stephen Lloyd, a curator at the National Portrait Gallery in Edinburgh, who gave a lecture in which he reattributed The Skating Minister and stated unequivocally that it was not by Raeburn, but Henri-Pierre Danloux, an obscure French artist who made lengthy visits to the Scottish capital in the 1790s to paint portraits of members of the French royal family in exile.

It was the opening salvo of a furore that shows no sign of dying down. At its heart is an oil painting which measures 76.2cm by 63.5cm. As Lloyd noted in the July issue of the Burlington Magazine, The Skating Minister is perceived not only as the most famous work by Henry Raeburn, it is "also one of the best-known Scottish paintings . . .

a quintessential picture from the 'golden age' of British art." Moreover it seems to sum up an idea of Scottishness, freezing comically in his time the moral rectitude of a Presbyterian minister, dressed in black gliding majestically across a frozen pond, ambivalently poised between heaven and hell.

So keenly has the Rev Walker been taken into the nation's affection that his image is ubiquitous - on ties, fridge magnets, mugs and other souvenirs sold in the National Galleries' shop. He is even said to have inspired Enric Miralles, the Catalan architect of the Scottish parliament, who took the minister's pose and adapted it for the idiosyncratically shaped windows at Holyrood.

Were it discovered that he was not painted by a fellow countryman, who could compute the damage to national pride?

Imagine if it were discovered that Sean Connery is English? Little wonder then that there are chauvinists who would rather the whole affair was swept under the carpet.

By questioning the provenance of such an iconic painting, Lloyd has opened the proverbial can of worms. A scholar of miniatures, he cuts an unlikely figure as provoker of controversy. Looking like Hugh Grant playing an adult Harry Potter, he wafts around the baronial interior of the Portrait Gallery like the laird of an ancient pile. In one room he points to Danloux's painting of Adam Duncan, first Viscount Duncan of Camperdown, in whose background is an officer, seen in profile, barking orders to sailors in the rigging. It is typical of Danloux, says Lloyd, reminiscent of the so-called "flying Mercury" pose favoured by skaters, including the Rev Walker. In contrast, there is nothing similar in Raeburn's oeuvre. If he did paint The Skating Minister, contends Lloyd, it must have been a one-off.

That Danloux, who was born in Paris, was in Edinburgh in the last decade of the 18th century is one of the few undisputed "facts" in this debate. Throughout the 1790s, when he was living in London, he made prolonged visits to the Scottish capital. His first visit, lasting between four and five months, was in 1796. He returned the following year and again in 1799. Throughout that decade he kept a diary which, says Lloyd, who has a copy to hand in his office, "is exceptionally interesting not only for the account of the emigre community, but also for his detached view of the British art world". Crucially missing from the diary, however, are entries for the period Danloux spent in Edinburgh, which would surely have settled one way or another the question of who painted The Skating Minister. This, acknowledges Lloyd, would be a very useful document to get one's hands on.

But he remains convinced of Danloux's authorship of the painting, which he suggests may be an homage not only to Raeburn but also to two other notable Scottish 18th century artists whom he admired: the caricaturist John Kay and Archibald Skirving, the only other contemporary portraitist who can be compared to Raeburn. "It may even be surmised that Skirving and Raeburn advised the Rev Robert Walker to have his informal skating portrait painted by the visiting French artist, " speculates Lloyd, "who would have been seen as a specialist in the portrayal of the figure in motion."

There is, though, no documentary evidence to back this up. Not that that is unusual in this peculiar case where paper trails lead to dead-ends and "killer facts" are conspicuous by their absence. As Lloyd says, "the circumstances surrounding the commissioning of the painting are unknown, and there is no record of this work in the art-historical literature or in biographical material relating to Walker before the 20th century". The painting emerged from "complete obscurity" in 1902, when we know it was seen by a restorer who suggested it was in need of some tender love and care. In 1914, Beatrix Scott put it up for sale at Christie's, hoping it might fetch 1000 guineas, but it failed to find a buyer, perhaps because of the imminence of war, perhaps - as Lloyd suggests - because of doubts about its provenance. The same year, James Greig, a leading expert on Raeburn, was the first to question

whether he painted it, setting a trend which continues to the present day. For instance, in 1997/98, when there was a major Raeburn exhibition in Edinburgh and London, at the forefront of which was The Skating Minister, the Burlington Magazine's reviewer, Alex Kidson, voiced his reservations, saying: "It has to be said that in the company it keeps here, it does not look like a Raeburn at all."

Nor have the National Galleries of Scotland, which eventually purchased the painting in 1949, been wholeheartedly supportive of Raeburn's cause. Though it has always been attributed to Raeburn, it was invariably with caveats. One catalogue entry stated that there was "no parallel in Raeburn's work" and it has been labelled a "puzzle painting". In 1972, when the Gallery published a history of its collection, no mention was made of the picture of the skating minister. Five years ago it was described as "in character quite unlike any other known paintings by Raeburn".

Furthermore, it was "the focus of more speculation concerning its date, the authorship and the identity of the sitter than any other single painting in the Gallery's Scottish collection". Timothy Clifford, the Director of the NGS, is apparently convinced The Skating Minister is by Raeburn but has yet to say so in public. Like the authentification of Hitler's fake diaries, misidentifying the painter of such an important and instantly recognisable picture could seriously damage one's career.

It is a point not lost on Duncan Thomson, formerly head of the Scottish National Portrait Gallery. The co-author of a book on The Skating Minister, he cuts a dashing figure with his wizard-like beard as he conducts me on a tour of the Galleries, intent on proving once and for all that Raeburn painted Rev Walker. Circumstantially, the evidence appears compelling.

"Raeburn, " says Thomson, "was not only a 'portrait painter in Edinburgh', but by far the pre-eminent portrait painter in Scotland." Born in Stockbridge in 1756 - thus making him a year younger than Rev Walker - he started out as a jeweller's apprentice and soon graduated to miniature painting.

But unlike Danloux, who was trained in the classical French tradition, Raeburn was self-taught. Crucially, as we shall see, he did not make preparatory drawings, as Danloux did, meticulously mapping out what he intended to do before applying paint to the canvas. Often Raeburn would over-reach himself, leading to mistakes. But when he was inspired, he was a virtuoso, imaginative, inventive, sui generis.

Raeburn's fallible method, says Thomson, can be seen most transparently in The Skating Minister's hat, around which is a pentimento, the trace of a painting beneath the finished work. In reproductions this can be hard to spot. On the original, however, it is blatantly obvious. Evidence of such changes, Thomson argues, which are the result of method and technique, is a common feature of Raeburn's work, his DNA, but alien to anything found in Danloux's.

"They are essentially the result of Raeburn's unpremeditated, empirical way of working, which is diametrically opposed to Danloux's method, which is so much based on preparatory drawing and careful preplanning that marked changes of this sort were never necessary. The ready visibility of such changes in Raeburn's work is the result of his thin, glazed application of paint, where the substance's potential for future transparency was evidently not given much thought. This is entirely different from Danloux's typically much more substantial paint film - in none of his works is there anything remotely like the frail, open handling of the areas of sky and hills in the portrait of Walker. These virtually abstract markings, quite roughly scribbled, are so loosely made that the trailing hairs of the brush consistently part to allow the much lighter coloured ground to show through over this entire

area of the picture."

Thomson speaks as a trained artist, which he is careful to highlight. The way paint is put on canvas, he insists, is crucial to our understanding of a painting's genesis. Across the world, experts are locked in scholarly combat over who painted what. Did Botticelli himself paint the Madonna which the National Galleries acquired recently or did it come from his workshop? Which of Rembrandt's works can be ascribed to the master and which to his apprentices? What does it matter anyway? Is a painting's worth - its intrinsic aesthetic value rather than the sum it may fetch should it be put up for sale - inextricably associated with who painted it? Would the Mona Lisa be worth less if we knew that it was not by Leonardo? Would the Auld Alliance be irreparably damaged if Scotland's best loved picture were by a Frenchman?

In answer to the last question, probably not. But judging by the audience reaction to lectures given by Lloyd and Thomson (the latter the former's erstwhile boss), it would be unwise to under-estimate the strength of public feeling and the sense of ownership they have for a painting. Both Lloyd and Thomson are no less passionate in presenting cases which would test any friendship. On the surface, it is an intellectual debate, devoid of emotion. But one need not probe very deeply before detecting intense rivalry. Were either of the protagonists able to prove definitively that The Skating Minister was painted by Raeburn or Danloux it would be a considerable coup.

For Thomson, it is all a matter of method.

"In the very broadest terms, " he says, "Raeburn summarises, while Danloux describes." There is a tell-tale vagueness about how Raeburn paints, he insists. "By contrast, Danloux is unrelenting in his pursuit of detail as he works, teasing out his descriptive marks towards all four boundaries of his canvas, irrespective of the size or subject." In order to illustrate what he means, he steers me to Raeburn's full-length painting of Major William Clunes, which hangs in the NGS near The Skating Minister. While an attendant looks anxiously on, he almost puts his finger on the canvas where what he calls Raeburn's "handwriting" can most clearly be seen. The effect is essentially abstract and imprecise, the artistic equivalent of doodling, examples of which abound in Raeburn's work and which can also be seen in the tufts of rough cut hair peeping out from under the hat of The Skating Minister.

To an untutored eye, it is temptingly convincing. But so too superficially are Lloyd's examples of the minister's pose which occur several times in Danloux's work but are nowhere to be seen in Raeburn's. "So what?" is Thomson's response as we flit past one of them in the National Portrait Gallery.

He remains baffled as to why so many connoisseurs have been persuaded by Lloyd's thesis. "When these stylistic features are taken in conjunction with a good provenance, " he concludes in an article for the Burlington Magazine which is yet to be published, "one can only ponder in mystification why this re-attribution of this unique little painting was ever attempted - why so many have soared to what [the Italian art critic] Giovanni Morelli called so many years ago, 'the higher spheres in the balloon of fancy'."

Mystified Thomson may be, but his is unlikely to be the last word on this irresolvable subject. To an informed eye, it all may seem obvious. But others, arguably as perceptive, disagree. Whichever line of enquiry one pursues, frustration seems to be the inevitable conclusion. If Danloux's diary is missing, so too is any record of Raeburn ever having painted The Skating Minister other than the testimony of the Rev Walker's great granddaughter. We do not know who commissioned it, nor for that matter do we know for certain that the figure actually is the Rev Walker and that the background is Duddingtson Loch. If it was in his possession at his death in 1808 at the age of 53, why was no mention made of it in his will? If it was by Danloux, why didn't he sign it, which was his normal practice? Could it be - playing devil's advocate for the moment - that it was painted neither by Raeburn nor Danloux?

That at least is something Lloyd or Thomson are not willing to countenance. Raeburn, says Duncan Thomson. Danloux, ripostes Stephen Lloyd. Any takers for not proven?